Hello,
You must have Pixel Cape, Armor, Helm, and Sword in your inventory (not equipped) along with 1000 Pixel Coins when you speak to Count Stan. You may need to have empty space in your USE inventory tab as well.
If you are unable to rank up to 5-Star Hero even with these conditions met, please submit a ticket to our Support team: https://support-maplestory.nexon.net